Process for the production of a thread having a variably modifiable thread profile and preferred application of the process

ABSTRACT

A process for producing threads by metal cutting when a more or less gradual change of the thread profile is desired in at least one of its sections. When producing a thread by turning, by milling or by turning and milling, the thread is cut in at least two successive operations with at least two different thread pitches and/or an offset value or a continuously changing pitch. The process is preferred for making possible a gradual change in the height and shape of the teeth or, as the case may be, the design of the thread bottom of a self-cutting thread to be progressively adapted to the curved outer profile of an artificial hip-joint socket.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a special process for the cutting production ofthreads, in which, at least in a part region of their extension, a moreor less sliding influence on or correction of the thread profile isdesired. Such a design can be advantageous for specific uses. Such apreferred use of the process is proposed.

2. Description of the Related Art

Threads are in widespread use as constructive elements in generalmechanical engineering. Threads are usually made cylindrical, and inaddition conical threads are also commonly used, for example foroil-field pipes. A large number of different thread profiles are knownand are laid down in standards. The said profile on a workpiece isusually invariable, that is to say the thread profile at the threadstart is identical to that at the thread end. Exceptions areconceivable, however, in which it could be advantageous to have a shapeof the thread profile formed from thread groove and thread tooth whichchanges in a flowing manner at least in a part region of the thread, forexample in order to make it easier to introduce a screw thread into anut thread.

However, special geometrical conditions with respect to the threadprevail primarily in threads on curved surfaces, such as occurparticularly in the case of screw-in artificial hip-joint sockets. Here,with regard to the outer shape of the shell body, for examplehypospherical, hemispherical or hyperspherical, conico-spherical,parabolic, toroidal, elliptic or similar geometries are known.Metal-cutting production processes for the threads of screw sockets ofthis type result, sometimes necessarily, in distortions of the threadprofile which vary in flowing manner and which in most cases are neitherintended nor desired. Particularly the use of thread teeth havingasymmetric flank angles, there is the phenomenon that, depending on thetilting direction of the resulting thread tooth, the tooth height fromthe socket equator towards the socket pole increases or decreases in aflowing manner, the result of this then being either much too large orvirtually stunted thread teeth at the near-pole thread start. In thefirst case, extremely large thread teeth led to the fact that very highforces become necessary in order to screw in the socket or the implantcannot be screwed in until there is full bone contact. In the secondcase, only very feeble primary fixation is to be achieved. In bothcases, there is the risk that the implant will work loose, and thiswould mean a further operation on the patient as a consequence.

SUMMARY OF THE INVENTION

The object was, therefore, to provide a hip-joint socket which can beproduced by metal cutting and with a thread located on the curved outersurface and having asymmetric flank angles of the said tooth and athread-tooth height adaptable along the thread extension, as well as aprocess for the production of such a special thread for this and otheruses.

The object is achieved, according to the invention, in that the threadgroove of a thread located on a curved outer surface is divided into atleast two strip-like part surfaces of different pitch, whereas the widthof the thread bottom, as measured in the radial projection, changes in aflowing manner along the extension of the thread, in such a way that thethread-tooth height which results in each case is adapted to theconstructive preconditions. Furthermore, the invention makes available aprocess for the production of such a special thread.

Thus, it is proposed to machine the said groove along the same contour,in the region provided for the adaptation of the thread profile, atleast in two production passes with at least two different pitches. Forthis purpose, the same tool or two or more different tools canoptionally be used. If only one tool is to be employed, its two flanksmust correspond to the shape of the two thread flanks. The inventionhere affords the possibility of choosing to use either a tool whichcorresponds in its profile to the profile of the thread groove at itsnarrowest point or a tool which is generally narrower than the threadgroove. In the second case, it is proposed, for the cutting of thethread, to include additionally an appropriately adapted offset valuebesides the use of at least two different pitches for the machiningpasses, in order to compensate the difference between tool width andthread-groove width. This procedure is particularly advantageousbecause, for example, a smaller tip rounding of the machining toolthereby becomes possible. As a result, the thread bottom can have afiner definition and be adapted more closely to the curved outer surfacedesired.

Furthermore, the invention offers the possibility of choosing to carryout the thread machining by means of more than two production passes orby means of two or more tools. If, for example, three production passesare employed, it is recommended to machine essentially one side of thethread groove in one of the production passes, essentially the otherside of the thread groove in a further production pass and essentiallythe groove bottom in a third production pass. At the same time, themaximum or minimum value of the pitch is used for the tools cutting thesides of the thread groove and the respective flanks of the threadteeth, whilst it is proposed to use a pitch located between these twovalues for the tool cutting essentially the middle of the thread bottom.With an increasing number of machining passes or tools, it is thuspossible, with a correspondingly small rounding of the respective tooltip, to avoid an unfavourably coarse rounding of the thread-tooth rootand to achieve an even better adaptation of the thread bottom to thedesired contour.

The process according to the invention can be used on various machinesby means of different cutting techniques, for example by means oflathe-turning, milling and turning or milling. In the lathe-turning ofthe threads, it is customary to machine the thread groove in a pluralityof passes by means of a lathe chisel. The production pass is thendesignated as a thread-cutting cycle. In the case of a single run, onlya little material is removed, but, for this purpose, the work can becarried out at high feed speeds. In contrast, in thread-milling, aproduction pass can consist of a single run, for which, however, as arule an expenditure of time somewhat greater in comparison withlathe-turning occurs as a result of the low feed speed required. Forcutting the thread according to the invention, moderncomputer-controlled machines, so-called CNC machines, are necessary. Thepath to be described by the respective tool in relation to the workpiecemust be entered in the corresponding CNC program. Furthermore, thedifferent pitch to be taken into account during cutting for therespective production pass is to be specified in the program. Forsynchronizing the production passes or tools, it is necessary to computeexactly the different starting points for the individual productionpasses or cutting cycles.

The computation of two such starting points will be explained by meansof an example of lathe production. A thread to be cut on a curved outersurface is to extend from z₃ =-8 to z₄ =-27. A pitch s₁ of 4 mm to becovered is assumed for a tool A. In view of the necessarysynchronization distance (usually 2 times thread pitch) of 8 mm, thestarting point for the thread-cutting cycle is set at z₂ =0, whilst theend point remains at z₄ =-27. A travel in z of 27 mm is thus obtained.To achieve a specific flowing adaptation of the thread-tooth height, fora second tool B the pitch s₂ to be taken into account was set at 4.2 mm.At the point z₄ =-27, the two tools are to be synchronous. The startingpoint z₁ for the tool B is then calculated as follows: ##EQU1##

If an offset value is used for the axial parallel shift of a single toolor if a position of two tools is not congruent at any point of thethread, this offset value is to be added or subtracted with the z value,determined according to the above formula, for the starting point.

It is proposed, as particularly advantageous, to combine the process,explained in more detail above, for the production of a thread having avariably modified thread profile with a process, on which the applicanthas already applied for a patent, for adapting the thread bottom tocurved surfaces. The said application (European Patent Application91250274.7, Publication No. 0,480,551 A1) presents a process, by meansof which the thread bottom can be adapted virtually perfectly to acurved shape, in that cutting tools having end angles differing inrelation to the thread bottom are used for production. These are movedon different paths, but with the same pitch relative to the workpiece.In the case of different (asymmetric) tooth-flank angles, the toothheights of threads of this type, the said tooth heights then normallychanging in a flowing manner, can be corrected to the desired values byadditionally moving the individual tools with different pitches relativeto the workpiece.

In a special version of the process hereby proposed, one or more toolsrun successively off the thread groove of a screw-in hip-joint socket ofthis type by the use of an offset and of different pitches, in such away that, in the middle of the thread bottom between the thread teeth ofconventional depth (for example, a depth of between 2.5 and 3.5 mm), afurther very small thread tooth (for example, with a depth of between0.2 and 1.5 mm, preferably between 0.3 and 1.0 mm) is formed. This typeof machining serves for providing a micro/macro thread on the outersurface of the screw socket, the macroteeth of normal size servingessentially for primary fixation and the small microteeth servingessentially for surface enlargement and therefore for secondaryfixation. By setting the geometrical cutting conditions from the numberof machining runs and their respective offset values or pitch values, itis possible, for such a micro/macro thread, to determine how far thetooth heights of both the macroteeth and the microteeth are either to bemaintained or to be varied in a flowing manner over the contour. Theproposed micro/macro thread achieves a highly reliable anchoring of theimplant in the bone structure, because the advantage of a larger threadpitch and of the resulting broader width and high load-bearing capacityof the bone structure located in the thread grooves is therebyassociated in a beneficial way with the advantage of surface enlargementand the accompanying reduction in the specific interfacial stressbetween the implant and bone structure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the presentinvention reference should be made by the following detailed descriptiontaken in with the accompanying drawings in which:

FIG. 1 shows a sectional and half-side representation of a somewhatexemplary embodiment of an artificial hemispherical hip-joint socketwith a conventionally produced screw thread.

FIG. 2 shows, in a mode of representation identical to FIG. 1, thesimplified and slightly exaggerated exemplary embodiment of anartificial hemispherical hip-joint socket with a screw thread producedin accordance with the invention.

FIG. 3 shows, in a mode of representation identical to FIG. 1, anartificial paraspherical hip-joint socket with a micro/macro screwthread produced in accordance with the invention.

FIG. 4 shows an artificial hemispherical hip-joint socket wherein thenear-perfect spherical outer surface is produced using five differentcutting tools in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding, the invention will be explained in moredetail below with reference to the four drawing figures.

FIG. 1 shows a sectional and half-side representation of a somewhatsimplified exemplary embodiment of an artificial hemispherical hip-jointsocket of medium size with a conventionally produced screw thread on ascale of approximately 2.5:1. In this, a value of 4 mm was chosen forthe thread pitch. The thread teeth have been shown slightly exaggerated,so that the details can be made more clearly visible. The hip-jointsocket (1) is provided with a bottom hole (2). Its inner contour isdivided into a spherical portion (3) and a cylindrical region (4). Thesix thread teeth (5, 6, 7, 8, 9 and 10) have an asymmetric profile.Their flank facing the pole of the socket has an angle of 0°, and theirflank facing the equator of the socket has an angle of 20°. This resultsin a tilting angle of the thread teeth of 10° in the direction of thesocket pole. This tilting direction of the thread teeth appearsparticularly beneficial for the intended use in a hip-joint socket,because, in view of the predetermined load direction, better conditionsfor the introduction of force into the human pelvis can thereby beachieved. Since the thread was cut in the conventional way with only onetool and with a constant pitch, the thread bottom (11) near the equatoris identical to the thread bottom (12) near the pole. The tilting angleof the thread teeth and the production process used for the threadresult in a flowing variation in the thread-tooth height. It can be seenclearly that the radially measured thread-tooth height h 2 near the poleis approximately twice as large as the thread-tooth height h 1 near theequator. Although flowing increases or decreases in the thread-toothheight may be perfectly desirable to a particular degree, especially inhip-joint sockets, and various arguments for this are also put forwardin the field, nevertheless such a pronounced enlargement of thethread-tooth height is extremely disadvantageous, because this involvesaction unnecessarily deep into the bone material. Furthermore,particularly in the case of self-cutting screw sockets, thread teeth ofsuch size at the near-pole thread start lead to very high screw-inforces, since, in the region of the abrupt increase in the thread-toothheight at the thread start, the cutting work during the screwing intothe pelvis is distributed over only a few cutting edges. Theintroduction of the hip-joint socket thereby may be impaired before fullbone contact is achieved.

FIG. 2 shows, in a mode of representation identical to that of FIG. 1,the simplified and slightly distorted exemplary embodiment of ahip-joint socket according to the invention with a screw thread. Thehip-joint socket (13) shown corresponds to that of FIG. 1 in its bottomhole (14), its inner contour consisting of a spherical portion (15) andof the cylindrical region (16) and the number of its thread teeth. Boththe near-equator thread tooth (17) with its shape and the amount of itsdepth h 1 and the thread bottom (23) of the adjacent thread groove areidentical respectively to the corresponding thread tooth (5) and to theadjacent thread bottom (11) of FIG. 1. As a result of the threadmachining according to the invention by means of two production passeswith a different pitch, the thread-tooth flank drawn at 0° being cut, asbefore, with a pitch of 4 mm, but, in contrast to this, the thread flankdrawn at 20° being cut with a pitch of approximately 4.16 mm, a radiallymeasured tooth height h 3 corresponding to the tooth height h 1 of thenear-equator tooth (17) is now produced for the near-pole thread tooth(22). It is also clear that the thread bottom of the near-pole threadgroove (24) is now altogether wider than the near-equator thread bottom(23) and has assumed a shape diverging from this and coming closer tothe spherical outer surface of the socket shell. A slightly eccentricedge (25) is indicated faintly in the thread bottom of the near-polethread groove (24), the said edge (25) extending along the thread grooveby virtue of the machining mode according to the invention and slowlythinning in its developed trend in the direction of the socket equator.The maximum height of this elevation is dependent on the geometry orrounding of the tool bit used and therefore, by appropriate selection,can be adapted to the constructive preconditions for the design of thethread bottom.

The example of a special version of the process according to theinvention is represented in FIG. 3 in the form of a paraspherical screwsocket having a micro/macro thread. In the way already known from thepreceding figures, the drawing figure shows a sectional diagram of ascrew socket (26) with bottom hole (27), the interior of which is formedfrom a spherical portion (28) with an adjoining cylindrical region (29).The screw socket is equipped on its outer surface with a macro thread,from which six teeth (30, 31, 32, 33, 34, 35) are obtained in thesectional diagram. In contrast to the shape of the sphere, in theexemplary embodiment shown a para-spherical contour is produced by meansof the thread bottom and, primarily in the region after the near-polethread start, has been provided with a certain contraction in relationto a spherical outer surface, in order to provide room here for the bonechips occurring when the socket is screwed into the bony bed. In thethread groove (38) near the socket edge, a not very tall tooth (36) ofthe microthread extending round approximately centrally in the threadgroove can be seen. In the near-pole thread groove (39), the tooth (37)of the microthread has increased in height. The greater increase inheight of the microtooth when the radial height of the macrotoothincreases only slightly results from machining by means of a singletool, two different pitches and a reciprocal offset value being used intwo machining runs. The process affords, in general, the possibility ofincreasing or adapting the number of machining passes of different pitchand/or reciprocal offset values or the pitch differences themselves, inorder to influence the geometrical cutting result. Consequently, forexample, the microthread can be set without further action to a constanttooth height.

The exemplary embodiment shown in FIG. 4 also corresponds in the chosenform of representation to the figures shown previously. This relates toa hemispherical screw socket (40), the spherical outer surface of whichwas reproduced virtually perfectly by employing a thread-producingprocess known from European Patent Application 91250274.7 by means ofthe use of five different cutting tools having the individual front-edgeangles of 2°, 14°, 25°, 35° and 44°. Both in the thread groove (52)located near the socket edge and in the near-pole thread groove (53),the desired curved contour is reproduced exactly in such a way that theridges (50) and (51) necessarily occurring as a result of the processare scarcely visible. For the exemplary embodiment shown, the saidprocess was combined with the process on which a patent is herebyapplied for, in order to influence the height of the thread teethinclined at 10° in the pole direction. At the same time, the respectivepitches for the five different cutting tools were selected so that thetool cutting the near-pole thread flank was operated with the smallestpitch and the tool cutting the thread flank pointing to the socket edgewas operated with the largest pitch. The pitch difference between thesmallest and largest pitch was set so that the radially measuredthread-tooth height h 1 of the thread tooth (44) rises to a slightlyincreased value h 4 over the thread teeth (45, 46, 47, 48) as far as thethread tooth (49), this rise in turn resulting from the fact that anattempt was made to bring the thread-tooth flank cut at an angle of 20°to an approximately uniform length. The screw socket with its bottomhole (41) and with its inner contour composed of the spherical portion(42) and of the cylinder (43) otherwise corresponds to the exemplaryembodiments shown previously.

It can be seen from the last exemplary embodiment that the processaccording to the invention is not restricted to producing a uniformthread-tooth height over the entire range of extension of the thread. Onthe contrary, it affords numerous possibilities of variation, in thateither exactly determinable increases or decreases in the thread-toothheights can be brought about by an appropriate setting of the differentthread pitches used or these modifications can be introduced partiallyin part regions of the thread extension or in a sliding manner by meansof the programming of a continuously changing pitch. Consequently, theinvention makes available a means of highly flexible use for the optimumadaptability of special threads, particularly of curved special threads.

What is claimed is:
 1. Process for the cutting production of a threadprofile on a body, said thread profile having a first thread flank, asecond thread flank, and a thread bottom between said flanks, saidthread profile gradually changing over at least a portion of said body,wherein said gradually changing thread profile is produced by a processcomprising:machining said first thread tooth flank of said threadprofile with at least one machining pass with a machining tool set at afirst pitch; and machining said second thread tooth flank of said threadprofile with at least one machining pass with a machining tool set at asecond pitch, wherein said first pitch differs from said second pitch.2. Process as in claim 1, wherein said machining tool for machining saidfirst tooth flank is a different tool from the tool for machining saidsecond tooth flank.
 3. Process as in claim 2, comprising machining apart of the thread bottom between the thread flanks with a thirdmachining tool, wherein the pitch used for said third machining tool isbetween the pitch used for the first machining tool and the pitch usedfor the second machining tool.
 4. Process for the cutting production ofa thread profile as in claim 1, wherein said process comprises:machiningsaid first thread tooth flank of said thread profile with at least afirst machining pass with a machining tool and at least a secondmachining pass offset from said first machining pass; and machining saidsecond thread tooth flank of said thread profile with at least a firstmachining pass with a machining tool and at least a second machiningpass offset from said first machining pass.
 5. Process for the cuttingproduction of a thread profile as in claim 4, wherein said machiningtool for machining said first thread tooth flank is a different toolfrom the tool for machining said second tooth flank.
 6. Process as inclaim 1 comprising:machining said first thread tooth flank of saidthread profile at a first pitch with at least a first machining passwith a machining tool and at least a second machining pass offset fromsaid first machining pass; and machining said second tooth flank of saidthread profile at a second pitch with at least a first machining passwith a machining tool and at least a second machining pass offset fromsaid first machining pass.
 7. Process as in claim 6, wherein saidmachining tool for machining said first tooth flank is a different toolfrom the tool for machining said second tooth flank.
 8. Process as inclaim 1, comprising:machining said first thread tooth flank of saidthread profile at a first pitch with at least a first machining passwith a machining tool and at least a second machining pass offset fromsaid first machining pass; and machining said second tooth flank of saidthread profile at a second pitch with at least a first machining passwith a machining tool and at least a second machining pass offset fromsaid first machining pass, wherein the starting point for said machiningof said second tooth flank is offset from the starting point formachining of said first tooth flank.
 9. Process for the cuttingproduction of a thread profile as in claim 8, wherein said machiningtool for machining said first thread tooth flank is a different toolfrom the tool for machining said second tooth flank.
 10. Processaccording to claim 8, wherein the starting point of one of the machiningpasses is used as the reference point to which the offset value(s) ofthe other machining p asses are added or subtracted.
 11. Process as inclaim 1, wherein the starting point for said machining of said secondthread tooth flank is offset from the starting point for machining ofsaid first thread tooth flank.
 12. Process for the cutting production ofa thread profile on a body, said thread profile having a first threadflank, a second thread flank, and a thread bottom between said flanks,said thread profile gradually changing over at least a portion of saidbody, wherein said gradually changing thread profile is produced by aprocess comprising:machining said first thread tooth flank of saidthread profile with at least one machining pass with a machining tool;and machining said second thread tooth flank of said thread profile withat least one machining pass with a machining tool set at a second pitch,wherein the starting point for said machining of said second threadtooth flank in relation to the starting point for machining of saidfirst thread tooth flank is offset and dependent upon the equation:##EQU2## where Z₁ =tool starting point for machining said second threadtooth flank Z₂ =tool starting point for machining said first threadtooth flank Z₄ =convergent tool point of both machining passes S₁ =firstthread pitch S₂ =second thread pitch.
 13. Process as in claim 1, whereinsaid machining tool for machining said first tooth flank is a differenttool from the tool for machining said second tooth flank.
 14. Processfor the cutting production of a thread profile on a body, said threadprofile having a first thread flank, a second thread flank, and a threadbottom between said flanks, said thread profile gradually changing overat least a portion of said body, wherein said gradually changing threadprofile is produced by a process comprising:machining said first threadtooth flank of said thread profile with at least a first machining passwith a machining tool and at least a second machining pass offset fromsaid first machining pass at a first pitch; and machining said secondthread tooth flank of said thread profile with at least a firstmachining pass with a machining tool and at least a second machiningpass offset from said first machining pass at a second pitch, whereinthe starting point for said machining of said second thread tooth flankin relation to the starting point for machining of said first threadtooth flank is offset and dependent upon the equation: ##EQU3## where Z₁=tool starting point for machining said second thread tooth flank Z₂=tool starting point for machining said first thread tooth flank Z₄=convergent tool point of both machining passes S₁ =first thread pitchS₂ =second thread pitch.
 15. Process for the cutting production of athread profile as in claim 14, wherein said machining tool for machiningsaid first thread tooth flank is a different tool from the tool formachining said second tooth flank.
 16. Process for the cuttingproduction of a thread profile on a body, said thread profile having afirst thread flank, a second thread flank, and a thread bottom betweensaid flanks, said thread profile gradually changing over at least aportion of said body, wherein said gradually changing thread profile isproduced by a process comprising:machining said first thread tooth flankof said thread profile with at least one machining pass with a machiningtool; and machining said second thread tooth flank of said threadprofile with at least one machining pass with a machining tool, whereinsaid thread bottom describes a curved outer surface on said body,wherein said thread profile is machined on different paths by means ofdifferent cutting tools having frontal cutting angles differing inrelation to each other, and wherein different pitches for said cuttingtools are used during the machining passes.
 17. Process for the cuttingproduction of a thread profile on a body, said thread profile having afirst thread flank, a second thread flank, and a thread bottom betweensaid flanks, said thread profile gradually changing over at least aportion of said body, wherein said gradually changing thread profile isproduced by a process comprising:machining said first thread tooth flankof said thread profile with at least one machining sass with a machiningtool. and machining said second thread tooth flank of said threadprofile with at least one machining pass with a machining tool, whereinsaid body is a hip-joint socket having a curved outer contour includinga socket rim and a socket pole, wherein said first thread flank facessaid socket pole and said second thread flank faces said socket rim, andwherein a smaller pitch is used for the machining passes of the toolwhich cuts the first thread flank than is used for the machining passesof the tool which cuts the second thread flank.
 18. Process as in claim17, wherein said curved outer contour is selected from the groupconsisting of conico-spherical, hypersperical, hemispherical,hypospherical, paraspherical, parabolical, elliptical, and toroidicol.19. A screw-in hip-joint socket having a curved outer contour providedwith a self-tapping thread for cement-less anchoring in the acetabulumby rotating about an axis, said thread having a profile comprising afirst thread flank, a second thread flank, and a thread bottom betweensaid flanks, said thread profile gradually changing over at least aportion of said self-tapping thread, wherein said gradually changingthread profile is produced by a process comprising:machining said firstthread tooth flank of said thread profile with at least one machiningpass with a machining tool; and machining said second thread tooth flankof said thread profile with at least one machining pass with a machiningtool in such a way that the thread bottom is divided into at least twostrip-like part surfaces of different pitch.
 20. Threaded hip-jointsocket according to claim 19, wherein the width of the thread bottom asmeasured in the axial direction changes gradually along the extension ofthe thread from the socket pole towards the socket rim.
 21. Threadedhip-joint socket according to claim 19, wherein said thread bottomdescribes a curved outer surface on said body, wherein said threadprofile is machined on different paths by means of different cuttingtools having frontal cutting angles differing in relation to each other,and wherein different pitches of cutting tools are used during themachining passes.
 22. Threaded hip-joint socket according to claim 19,wherein said micro-thread tooth is at least 50% smaller than saidself-tapping thread teeth.
 23. Screw-in hip-joint socket having a curvedouter contour provided with a self-tapping thread for cement-lessanchoring in the acetabulum by rotating about an axis, said threadhaving a profile comprising a first thread flank, a second thread flank,and a thread bottom between said flanks, said thread profile graduallychanging over at least a portion of said self-tapping thread, whereinsaid gradually changing thread profile is produced by a processcomprising:machining said first thread tooth flank of said threadprofile with at least one machining pass with a machining tool; andmachining said second thread tooth flank of said thread profile with atleast one machining pass with a machining tool in such a way that thethread bottom is divided into at least two strip-like part surfaces ofdifferent pitch, further comprising a micro-thread tooth in the threadbottom formed between two self-tapping thread teeth, wherein saidmicro-thread tooth is smaller in height than said self-tapping threadteeth.
 24. Threaded hip-joint socket having a curved outer contourhaving a pole, a rim, a self-tapping thread for cement-less anchoring inthe acetabulum, said thread having a profile comprising a first threadflank, a second thread flank, and a thread bottom between said flanks,said thread bottom comprising strip-like part surfaces generallydefining the curved outer contour of the hip-joint socket shell, saidstrips provided angularly in steps and extending parallel to one anotherand obliquely relative to the development of the thread bottom, thethread tooth being inclined towards the pole of the hip-jointsocket,wherein the axial width of the thread bottom, as measured in theradial projection, varies gradually along the thread from the socketpole towards the socket rim, and wherein the strip-like part surface ofthe thread bottom extending along the thread-tooth flank facing thesocket pole has a smaller pitch than the strip-like part surface of thethread bottom extending along the thread-tooth flank facing the socketrim.
 25. A threaded hip-joint socket provided with a self-tapping threadfor cement-less anchoring in the acetabulum, said thread having aprofile comprising a first thread tooth flank, a second thread toothflank, a thread bottom between said flanks, said thread bottomrepresenting a curved outer contour of the socket body, said threadprofile gradually changing over at least a portion of said body, whereinat least a portion of the thread is machined using a continuouslychanging pitch.
 26. Process for the cutting production of a threadprofile on a body, said thread profile having a first thread flank, asecond thread flank, a thread bottom between said flanks, said threadprofile gradually changing over at least a portion of said body, whereinsaid gradually changing thread profile is produced by a processcomprising:machining said first thread tooth flank of said threadprofile with at least a first machining pass with a machining tool andat least a second machining pass offset from said first machining pass;and machining said second thread tooth flank of said thread profile withat least a first machining pass with a machining tool and at least asecond machining pass offset from said first machining pass, whereinsaid machining tool for machining said first thread tooth flank is adifferent tool from the tool for machining said second tooth flank,wherein the cutting tool has a rounded tip; and wherein the cutting toolrounded tip is smaller than the thread-groove width, wherein thestarting point for said machining of said second thread tooth flank inrelation to the starting point for machining of said first thread toothflank is offset and dependent upon the equation: ##EQU4## where Z₁ =toolstarting point for machining said second thread tooth flank Z₂ =toolstarting point for machining said first thread tooth flank Z₄=convergent tool point of both machining passes S₁ =first thread pitchS2 =second thread pitch.